The present invention relates to a power steering apparatus for generating a steering assisting force to be imparted to a steering mechanism by hydraulic pressure produced by a pump driven by an electric motor.
A power steering apparatus is known which assists the steering of a steering wheel by supplying a hydraulic fluid from an oil pump to a power cylinder connected to a steering mechanism. The oil pump is driven by an electric motor so that a steering assisting force corresponding to the rotational speed of the electric motor is generated from the power cylinder.
A torsion bar which produces torsion in correspondence with the direction and magnitude of the steering torque applied to the steering wheel, as well as a hydraulic control valve whose opening degree changes in correspondence with the direction and magnitude of the torsion of the torsion bar, are incorporated in a steering shaft. This hydraulic control valve is interposed in a hydraulic system between the oil pump and the power cylinder, and causes a steering assisting force corresponding to the steering torque to be generated from the power cylinder.
Control of the driving of the electric motor is effected on the basis of, for instance, angular velocity of steering of the steering wheel. Namely, the angular velocity of steering is determined on the basis of an output of a steering sensor provided in association with the steering wheel, and a target rotational speed of the electric motor is set on the basis of this angular velocity of steering. A voltage is supplied to the electric motor so that this target rotational speed will be attained.
Specifically, as shown in FIG. 10, the target rotational speed is set so as to increase up to a maximum rotational speed as the angular velocity of steering increases. Consequently, a steering assisting force corresponding to the angular velocity of steering is generated.
In a case where the steering wheel is continuously steered without driving, and in a case where a steering-maintained state continues in which the steering wheel is held with-torque being applied thereto, the electric motor assumes a continuously energized state, and there is a possibility of this electric motor assuming a state of an abnormally high temperature and undergoing a failure. Accordingly, a temperature sensor for detecting the temperature of the electric motor is provided, and the maximum rotational speed of the electric motor is limited on the basis of an output of this temperature sensor.
Specifically, as shown in FIG. 11, the overheating of the electric motor is prevented such that the higher the temperature of the electric motor, the lower the maximum rotational speed of the electric motor is set.
Namely, when the temperature of the electric motor becomes high, the maximum rotational speed of the electric motor is limited, as shown by the two-dot chain line in FIG. 10.
With the above-described configuration, however, as shown in FIG. 12, when the temperature of the electric motor is high, the steering effort (the force applied to the steering wheel by the driver) increases sharply in conjunction with an increase in the angular velocity of steering. For this reason, since the steering resistance suddenly becomes large at the time of sudden steering for avoiding an emergency, the driver feels that the steering has suddenly become heavy. This not only deteriorates steering feel but also hinders proper steering.
On the other hand, as another problem, depending on the steering situation there are cases where the driver feels that the steering is light or heavy owing primarily to the inertia of the electric motor, and discomfort of steering thereby occurs. This problem can be alleviated by using a low-inertia rotor, by adopting a three-phase bipolar motor drive system, or by making nonlinear the target rotational speed with respect to the angular velocity of steering.
However, these solutions make the electric motor and its drive circuit expensive or makes inevitable an increase in the capacity of the ROM of a microcomputer, so that an increase in cost is consequently unavoidable.